1. Field of the Invention
The present invention relates to an ultrasound image diagnostic apparatus.
2. Description of Related Art
Ultrasound diagnosis provides information about the pulsation of a heart or movements of a fetus just by a simple operation that involves application of an ultrasound probe to the surface of a body. The ultrasound diagnosis, which is highly safe for human bodies, is available in repeated inspections.
In such an ultrasound diagnostic imaging technique, a satisfactory image having a high contrast can be produced from the harmonic frequency components (for example, frequency 2f0 or 3f0) of the fundamental frequency component (frequency f0) of a transmission signal. Such an imaging technique is referred to as tissue harmonic imaging.
The main cause of these harmonic components is nonlinear distortion generated during propagation of ultrasound through the body of a subject. In other words, ultrasound signals applied to the living body is distorted during propagation through the tissue due to a nonlinear response from the tissue, which increases harmonic components. As a result, the echo signal contains the component of the frequency 2f0, which is twice the fundamental frequency f0, and the component of the frequency 3f0, which is three times the fundamental frequency f0.
In the tissue harmonic imaging, one of the traditional methods for extracting harmonic components from an echo signal is filtering. This method involves the extraction of the harmonic component of the frequency 2f0 from a reception signal with a bandpass filter having a central frequency of, for example, 2f0. Another example is a pulse inversion technique. The pulse inversion technique involves the transmission of a signal of a first transmission waveform and another signal of a second transmission waveform having the reverse polarity to the polarity of the first transmission waveform at a certain time interval and phasing addition of their echo signals to suppress the fundamental frequency component and thus emphasize the second harmonic component. The suppression of the fundamental frequency component requires the transmitter that transmits a first transmission waveform and a second transmission waveform having the reverse polarity to the polarity of the first transmission waveform to have a high positive-negative symmetricity of an excitation signal.
Unfortunately, harmonic components of an ultrasound signal, which have a higher frequency than the fundamental component, are more vulnerable to attenuation during propagation and have low transmission or penetration of the eco signal from deep sites. A lower central frequency f0 of fundamental waves can make harmonic components less vulnerable to attenuation and thus improve penetration, but reduces resolution, as is known in the art.
Of these two methods, the filtering method, which involves a uniform cutting of the low-frequency region without any distinction between the fundamental and harmonic waves, has a significant impact on the cutting, thus produces a narrow bandwidth after extraction and is generally inferior in the quality of the resulting images to the pulse inversion technique. Accordingly, the pulse inversion technique is the mainstream except for low-priced devices.
In recent years, methods for enhancing the penetration while maintaining the resolution have been proposed in the pulse inversion technique. Such proposals include the use of subharmonics of a frequency lower than that of the second harmonic (refer to JP 2002-301068A) or the generation of harmonics of a frequency in a range from f0 to 2f0 (twice the frequency of f0) (refer to JP 2003-310609A). Unfortunately, these methods still require a transmitter to have a high positive-negative symmetricity of an excitation signal.
In addition, there is an increased demand, not for the traditional single-frequency transmission, but for transmission of more complex temporal waveforms containing multiple frequencies or involving frequency transition. Such complex temporal waveforms cannot be controlled by low-priced transmitters capable of only transmitting signals having three levels of voltage (+HV/GND/−HV) or five levels of voltage (+HV/+MV/GND/−MV/−HV) and require adoption of a transmitter capable of driving multiple levels of voltage or waveform shaping at a peripheral circuit. Such a tendency further precludes reductions in cost and sizes of such transmitters.
As a result of extensive studies, the present inventor has successfully found a method for solving the above-mentioned problems. More specifically, the inventor can provide an ultrasound diagnostic apparatus capable of acquiring harmonic images with significantly high resolution, even with a transmitter capable of addressing only five or less levels of voltage, by setting the peak intensity of power spectrum at the frequency of a transmission signal to an appropriate value (See JP 2014-168555A).
The ultrasound diagnostic apparatus disclosed in JP 2014-168555A is usable as it can acquire high-resolution images and has high penetration. Unfortunately, the disclosed apparatus tends to have reduced resolution for deep sites. There is a demand for maintaining a high resolution and a high signal-to-noise ratio for shallow sites and improving resolution for deep sites to acquire highly uniform harmonic images for the shallow to deep sites.
One of the methods for generating harmonic images for deeper sites is to increase driving voltage. Such an increase in driving voltage requires a high-capacity power supply and an ultrasound image diagnostic apparatus having a high voltage resistance. This configuration results in an increase in size and cost of the apparatus and cannot be achieved by a low-priced small apparatus. The increase in voltage also increases the risk of depolarization or insulation breakdown of the piezoelectric elements of an ultrasound probe. Thus, a method that does not involve an increase in voltage is required for the visualization of deeper sites.
One of the methods for enhancing a signal-to-noise ratio that do not involve an increase in voltage is a pulse compression technique. The pulse compression technique involves the transmission of long pulses, such as coded signals or chirp waves, and the application of a matched filter to received signals to acquire short pulses. Such a pulse compression technique is, in principle, applicable only to fundamental imaging, not to harmonic imaging.
The harmonic imaging uses a method for generating a long pulse just by concatenating traditional transmission waveforms. Unfortunately, a excitation time exceeding the length of 1.5 waveforms at the lower limit frequency of the −20 dB bandwidth of an ultrasound probe reduces the resolution of images which are acquired by generating long pulses from transmission signals, thus precluding the acquisition of ultrasound images that satisfy the above-mentioned requirements.